The paucity of regeneration in the mammalian CNS continues to present a formidable challenge. The neuron must be capable of reinitiating and sustaining growth of its severed processes and the environment must be supportive for the growing fibers. Evidence that growth associated proteins are present in regenerating adult amphibian optic nerve and adult rabbit peripheral nerve but absent in adult rabbit optic nerve argues that the mammalian CNS neuron may not be capable of regeneration after injury. Nevertheless, recent studies show that provision of supportive terrain, a peripheral nerve segment, promotes regrowth of CNS neurons. The growth cone and its interaction with the environment has been most easily and extensively studied in tissue culture. Few culture studies, however, have assessed the age dependence of regenerating processes from PNS neurons or investigated growth cones of CNS neurons. We proposed here experiments to study the mechanisms underlying the differences in the rate of translocation of growth cones from perinatal and postnatal sympathetic neurons. Specific questions include: 1) What ultrastructural features are present in growth cones translocating at different rates that give clue to the mechanisms(s) underlying those differences? 2) Do differences exist in the rate of movement or quantity of membranous elements in the fast component of axonal transport? 3) Do differences exist in the transport of actin along the neurite and its distribution in the growth cone? 4) What environmental factors (substratum or medium) promote increased neurite extension, especially from older neurons? Secondly, culture conditions will be sought to grow several types of CNS neurons. Questions of interest include: 1) Can neurite extension be shown to be age dependent for CNS neurons? 2) Are there differences in CNS cones compared with those of sympathetic neurons or other types of CNS neurons that may vary in regenerative capacity? 3) What conditions (substratum or medium) foster the growth of CNS neurites? Techniques to be used are video-enhanced and intensified microscopy, time lapse and computer assisted measurement of neurite extension, immunocytochemistry, and electron microscopy. Answers to these questions will increase our fundamental knowledge of the biology of the growth cone and regulation of its forward movement. A better understanding of factors promoting growth cone function will increase our ability to improve regeneration and may in time be translated into practical strategies for helping patients with CNS injury.